Technical Field
The present invention relates to a control apparatus of a motor which drives a feed shaft, a spindle, or the like in a machine such as a machine tool, and in particular, to monitoring of a temperature of wiring of the motor.
Related Art
In many motors, as disclosed in Japanese Patent No. 4135437, a wiring temperature detection element 7 such as a thermostat and a thermistor is provided at a coil end 3 of a wiring of a stator 6 (FIG. 1). A wiring temperature is monitored by these temperature detection elements, and when the temperature becomes a particular temperature, a current to the motor is limited or electricity application is discontinued, to protect the wiring from burnout.
This method ensures reliable protection because the temperature of the wiring can be directly monitored. In addition, because the electricity application can be continued until the temperature of the motor becomes a temperature very close to the heat resistant temperature of the motor, the characteristic of the motor can be utilized to a maximum degree.
In addition to the above, other methods of burnout protection of the wiring are known such as that shown in JP 2008-113477 A in which the temperature detection element is not used, a change of the wiring temperature is estimated based on a current during driving of the motor or a torque command value which is in a proportional relationship with the current, and the current to the motor is limited or the electricity application is discontinued when the estimated value becomes a particular value.
As this method does not use the temperature detection element, the method is advantageous in cases where the temperature detection element cannot be provided because there is no spatial margin in the structure of the motor or where the cost is to be reduced.
The method of estimating the change of the wiring temperature may be represented by a simple model as shown in a block diagram of FIG. 2. The estimation equation is represented by the following equation:ΔT′(n)=β×{(α×Tin)2−ΔT′(n−1)}+ΔT′(n−1)  (Equation 1)
Here, ΔT′ represents an estimated value of the change of the wiring temperature, α represents a coefficient which determines a saturated value of the estimated value of the change of the wiring temperature, β represents a coefficient which determines a time constant of the estimated value of the change of the wiring temperature, and Tin represents a command value or detected value of a current or a torque command value which is in a proportional relationship with the current. The index (n) represents the number of detection periods. That is, ΔT′ (n) represents the estimated value of the change of the wiring temperature at an nth detection period. Equation 1 corresponds to a wiring temperature change estimation unit 20 of FIG. 2. Equation 1 is an equation for calculating the estimated value of the change of the wiring temperature at the nth detection period based on the estimated value of the change of the wiring temperature at an (n−1)th detection period and the current command value or the like.
A relationship between ΔT′ which is an estimated value and ΔT which is the actual change of the wiring temperature is shown in FIG. 3. When β is adjusted, a slope of ΔT′ changes as shown by reference numeral 70, and when α is adjusted, an upper limit value of ΔT′ changes as shown by reference numeral 71. By adjusting these two parameters, it becomes possible to calculate the estimated value adapted for the thermal characteristic for each motor type. Tin is squared because the model calculates an amount of generated heat due to copper loss calculated by (square of current)×(resistance).
In the method of providing the temperature detection element on the wiring as described above, the motor must be designed in consideration of the space for placing the temperature detection element and a wiring method. In addition, an additional cost would be required for adding the temperature detection element. For these reasons, the method is not suited for small-size, low-cost motors.
In the method of estimating the change of the wiring temperature based on the current during driving of the motor or the like without the use of the temperature detection element, the temperature of the wiring cannot be known as an absolute value. Normally, a threshold for discontinuing the electricity is set assuming a state of a high environmental temperature.
Specifically, when a heat resistance temperature of the wiring of the motor is assumed to be 140° C. and the maximum environmental temperature is assumed to be 40° C., the motor is designed such that the electricity application is discontinued when the temperature is increased by 100° C. (=140° C.−40° C.).
When the motor is designed in the above-described manner, when the actual environmental temperature is 20° C., the temperature where the wiring temperature is increased by 100° C. and the electricity application is to be discontinued is 120° C. (=20° C.+100° C.). In other words, the electricity application is discontinued in a state where there still is a margin of 20° C. for the wiring of the motor (refer to FIG. 4).
On the other hand, when the motor is used in a situation with the actual environmental temperature of 60° C., the electricity application is not discontinued until the actual wiring temperature is increased from 60° C. by 100° C., that is, to 160° C. (=60° C.+100° C.). Because of this, the temperature exceeds 140° C., which is the original intended temperature where the protection operation is to be started, by 20° C. (refer to FIG. 5), which results in possibility of damage to the wiring and failure of the motor.
In order to solve these problems, the wiring temperature must be monitored as an absolute value corresponding to the actual temperature, and not as the change from a certain unknown temperature. In addition, it is necessary that the absolute value not be obtained directly from the wiring temperature, but be estimated from other physical parameters.